For example, a solenoid is generally used for an actuator of a cylinder, an electromagnetic valve and the like. For example, JP10-19156A discloses a current control device that controls an exciting current of a solenoid of an electromagnetic valve by a pulse width modulation (PWM) signal.
In JP10-19156A, the exciting current is periodically varied in a dither period having a length of several times a pulse period of the PWM signal so as to create small oscillation of a spool of the electromagnetic valve, thereby to reduce an appearance of hysteresis characteristics caused by the static friction of the spool.
In JP10-19156A, a duty ratio of the PWM signal for generating the exciting current as a target is set according to each dither period. Therefore, if the target is changed during the dither period, this change is reflected on the duty ratio of the PWM signal when the next dither period elapses. Namely, the renewing of the duty ratio of the PWM signal delays from the timing where the target is changed. Therefore, an operation responsiveness of a movable core driven by the solenoid is low.